<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- this -->
    <button id="btn">btn</button>
    <script>
        var btn =document.getElementById("btn")
        // btn.onclick =function(){
        //     setTimeout(function(){

        //         console.log(this.id);    //undefined
        //     }.bind(this),1000)
        // }
        // 1 bind   2 箭头函数
        btn.onclick =function(){
            setTimeout(()=>{
                console.log(this.id);
            })
        }
    </script>
</body>
</html>